Kirk Wallace wrote: > On Mon, 2008-04-28 at 19:54 -0400, John Kasunich wrote: > ... snip >> Using a schmidt trigger to clean up the output is good. However, there >> is one potential problem. The two stage charge pump can produce more >> than 5V, which the '14 might not like. Maybe a clamp diode to the >> supply rail? Or maybe use just one stage - remove C2, short D3. You'd >> have to make sure that one stage generates enough voltage to reliably >> trigger the '14. > ... snip >> John Kasunich > > You are right. I got 5.5 Volts on the pump output going into the first > buffer. I figured it might be okay. I like the clamp diode idea. > > I see the datasheet indicates that I am at the max. TI's LS14 shows a > zener on the input already, so I guess I don't really know how hard I am > driving the input. > After I wrote my reply I looked closer at the schematic and saw the how big your caps are. I also noticed that you are using an LS14. I used much smaller caps, probably 0.1uF for the filter and 0.01uF or smaller for the pumps. I used proportionally higher resistors to get the time constants I wanted, but the net result is less loading on the '14 sections that are driving the pump.
In your case, the input current requirements of the LS14 make it interesting. IIRC, LS inputs go high when open - you need to make sure your load resistor R5 is low enough to ensure a logic zero when the pump is off. If you've socketed the chip, I'd strongly consider replacing it with CMOS one, like an HC or HCT. Since the CMOS parts have very high DC input impedance, you could put 10K to 50K between the top of R5 and the input to the chip. That would greatly limit any current that might flow into internal protection diodes, and make the clamp issue go away. The tradeoff between HC and HCT is driven mostly by the input from the PC (pin 5). HC is better for the output section, since I think the HC thresholds are about mid-rail. HCT thresholds are lower, and are a good choice if the parport might be driving only 3.3V. Regards, John Kasunich ------------------------------------------------------------------------- This SF.net email is sponsored by the 2008 JavaOne(SM) Conference Don't miss this year's exciting event. There's still time to save $100. Use priority code J8TL2D2. http://ad.doubleclick.net/clk;198757673;13503038;p?http://java.sun.com/javaone _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
